1. Field of the Invention
The present invention relates to a reversible thermal recording method and a reversible thermal recording apparatus therefor, and particularly to a reversible image forming and erasing method using a reversible thermosensitive recording material having a recording layer in which a color image is repeatedly formed and erased by controlling heat energy applied to the recording layer and a cooling speed after the heating, and to a reversible image forming and erasing apparatus therefor.
2. Discussion of the Related Art
Currently, hard copies are obtained, for example, by the following methods:
(1) toner images are formed and fixed on a recording material by, for example, electrophotography or the like; PA1 (2) ink images are formed and fixed on a recording material by, for example, a printing method such as offset printing methods and ink jet printing methods, a thermal transfer recording method or the like; and PA1 (3) visible images such as dye images are formed on a recording material such as a thermosensitive recording material by, for example, a method such as a thermal recording method. PA1 providing a reversible thermosensitive recording material including a recording layer which is formed overlying at least one side of a substrate and which includes an electron donating coloring agent and an electron accepting color developer, the recording layer achieving a colored state when heated to a temperature not lower than an image forming temperature and then cooled at a speed not slower than a cooling speed (a), and the recording layer in the colored state achieving a non-colored state when heated at a temperature not lower than an image erasing temperature but lower than the image forming temperature or when heated at a temperature not lower than the image forming temperature and then cooled relatively slowly compared to the cooling speed (a); PA1 first heating the recording layer such that the recording layer achieves the non-colored state; PA1 then imagewise heating the recording layer such that an image of the colored state is formed in the recording layer; and PA1 then cooling the recording layer at a speed not slower than the cooling speed (a) to maintain the color image in the recording layer. PA1 an image erasing device which heats the recording layer of the reversible thermosensitive recording material mentioned above so as to achieve a non-colored state; PA1 an image recording device which imagewise heats the recording layer to form an image of the colored state in the recording layer; PA1 a cooling device which cools the recording layer at a speed not slower than the cooling speed (a) to maintain the color image in the recording layer; and PA1 a feeding device which feeds the recording material such that the recording material is successively processed by the image erasing device, the image recording device and the cooling device in this order.
The consumption of these recording materials is rapidly increasing because copiers, facsimile machines, and printers which are used as output terminals of computers, are increasing. This causes social problems such as environmental disruption and environmental pollution. Reversible recording materials which can repeatedly form and erase images attract considerable attention because they can stop the increase of or decrease the consumption of these recording materials.
For example, transparent/opaque type reversible thermal recording materials have been disclosed which can reversibly form and erase images by achieving a transparent state and an opaque state utilizing change of light scattering properties of a polymer film in which organic crystalline particles having low molecular weight are dispersed (e.g., Japanese Laid-Open Patent Publication No. 55-154198). These transparent/opaque type reversible thermosensitive recording materials have been practically used as displays of, for example, magnetic cards and the like. However, the images formed in the displays are white images on a colored background such as black or blue, or on a light reflective background such as aluminum plates or aluminum-evaporated materials, and therefore the images are not preferable because they are very different from the images of the hard copies obtained by the methods mentioned above.
A recording apparatus which is useful for the transparent/opaque type reversible thermal recording method and in which a cooling device is provided after an image recording section is disclosed in Japanese Laid-Open Patent Publication No. 8-90934. However, this cooling device is provided only to shorten the time during which a recording layer changes from a transparent state to an opaque state. In this case, even when image erasing and forming operations are repeatedly performed and therefore the temperature of the cooling device increases, i.e., the recording layer is gradually cooled, the opaque state can be securely obtained although the cooling speed is prolonged.
Color/non-color type reversible thermosensitive recording materials (hereinafter referred to as color/non-color type recording materials) have been proposed which can reversibly form and erase color images on a white background using a composition of a leuco dye and a color developer which can reversibly achieve a colored state and a non-colored state (e.g., Japanese Laid-Open Patent Publication No. 5-124360). The coloring and decoloring of the color/non-color type recording materials can be controlled by controlling a heating temperature and a speed of cooling after the recording materials are heated. The colored state can be achieved by heating a recording layer including a leuco dye (coloring agent) and a color developer to an image forming temperature at which the leuco dye and the color developer are melted and mixed with each other, and then rapidly cooling the recording layer. At this point, if the recording layer is gradually cooled, good image density cannot be obtained. The color/non-color type recording materials are very different from the transparent/opaque type reversible thermosensitive recording materials in this respect. The non-colored state can be achieved by heating the recording layer at a temperature slightly lower than the coloring temperature.
In the transparent/opaque type recording materials, whether the recording materials achieve the transparent state or the opaque state depends on a heating temperature and does not depend on a cooling speed after the heating. Therefore, images can be recorded in a recording layer of the transparent/opaque type recording material by heating the recording layer with a thermal printhead to a relatively high temperature (an image forming temperature) for a moment, and the images can be erased by merely heating the recording layer with, for example, a hot stamp at a temperature slightly lower than the image forming temperature.
On the contrary, in the color/non-color type recording materials whether the recording materials achieve the colored state or the non-colored state depends on both a heating temperature and a cooling speed after the heating. Good color images cannot be obtained unless the recording layer is rapidly cooled after the recording layer is heated to form images therein. When reversible themosensitive recording materials are repeatedly used, new images are typically formed in the recording layer soon after former images formed therein are erased. In this case, the recording layer tends to be relatively hot when new images are formed because the recording layer is heated to erase the former images, and therefore the recording layer cannot be rapidly cooled after the new images are formed, resulting in formation of images having poor image density.
To avoid this problem, it has heretofore been considered that the recording layer of the color/non-color type recording materials has to be cooled by some method during the time between an image erasing operation and a subsequent image forming operation. If the recording layer is cooled by prolonging the time interval between the image erasing operation and the subsequent image forming operation, or by locating the image forming section far apart from the image erasing section, another problem which occurs is that it takes a long time to form new images or the image forming and erasing apparatus becomes large. There is another method for increasing the cooling speed in which a recording material is cooled with, for example, a metal plate after the image erasing operation. However, when image forming and erasing operations are continuously performed, the temperature of the metal plate increases, resulting in occurrence of the poor image density problem mentioned above.
In addition, another method is proposed in which a cooling device is provided between an image erasing section and an image forming section and in which a heating device such as a hot stamp is used as an image erasing device. In this case, when the cooling device is placed near the hot stamp which supplies high heat energy for erasing images and when images are continuously formed and erased, the temperature of the cooling device increases, resulting in occurrence of the poor image density problem mentioned above.
Further, when a cooling device is provided between an image erasing section and an image forming section, and when a recording layer is excessively cooled, relatively high heat energy is needed when images are recorded in the recording layer.
Because of these reasons, a need exists for a reversible thermal recording method which is useful for color/non-color type reversible thermosensitive recording materials and by which good images can be stably formed without applying relatively high heat energy to the recording material even when images are repeatedly recorded and erased for a long time.